Resolved via the last post here: Internal Server error No error code - #5 by Immuc
I ran the following from within my Nextcloud directory:
sudo apt install php7.4-fpm
sudo -u www-data php occ maintenance:mode --on
chown -R www-data:www-data /opt/ibp
chmod -R 770 /opt/ibp
sudo -u www-data php occ maintenance:repair
sudo -u www-data php occ db:add-missing-columns
sudo -u www-data php occ db:add-missing-indices
sudo -u www-data php occ maintenance:update:htaccess
sudo -u www-data php occ maintenance:mode --off
sudo systemctl restart apache2
I had not had php7.4-fpm installed (my PHP version is php7.4), as that was not required for Nextcloud 20 and before. That should have been checked by the upgrader or admin screen, the same as how there was a warning about MySQL 5 not being supported.